This invention relates generally to the field of medical ventilators, and more particularly to a ventilator having a valve which controls the flow of gases to a patient during artificial respiration and the means by which the valve is controlled.
Medical ventilators have been developed to provide artificial respiration to patients whose breathing ability is impaired. Typically, a ventilator will deliver a breath to the patient from a pressurized source of gas. Flow to the patient during inspiration is governed by a flow control valve. When the flow control valve opens, pressurized gas is introduced to the patient's lungs. After the flow control valve closes, ending the inspiration phase of the breath, the patient's respiratory gases are vented to the atmosphere through an exhalation valve, which opens after inspiration is completed and closes before the next inspiration phase begins.
Previous ventilators have been capable of operating in several modes so that the degree of support that the ventilator provides to the patient's natural breath patterns can be varied. At one extreme, the ventilator can provide fully controlled ventilation in which the ventilator has complete control over when the breath is delivered and the volume of gases received by the patient during each breath. In the fully controlled mode, all of the flow parameters are preset by an operator in accordance with the particular needs of the patient. The predetermined flow parameters include the tidal volume, or the volume of gas which is inhaled by the patient during each breath; the breath rate, which is the number of breaths per minute; the peak flow rate, which is the maximum flow rate of the gas delivered to the patient during inspiration; and the breath profile, which is the shape of the curve in a flow rate versus time graph. The most significant of these parameters is the tidal volume, since, depending on size and age of the patient, the tidal volume can vary significantly. Thus, the fully controlled ventilation mode is often called "volume controlled" ventilation.
At the other extreme, the ventilator can be programmed to permit "spontaneous" breathing by the patient. During the spontaneous breathing mode, the breath rate, the tidal volume and other flow parameters are not preselected. The inspiration and expiration phases of each breath are commenced in response to effort by the patient. In between the "volume controlled" and the "spontaneous breath" modes, various degrees of ventilator supported respiration are available.
One of the problems associated with previous ventilators has been that during the volume controlled mode, it is extremely difficult to precisely measure the volume of flow during inspiration so as to determine when the preselected tidal volume has been delivered, and thus at which point the flow control valve must be closed. Previous ventilators, such as that described in U.S. Pat. No. 4,527,557, have utilized a microcomputer controller which receives input regarding the desired flow parameters and compares these preselected parameters with actual flow conditions to control the opening and closing of the flow control valve.
In particular, a "closed loop" control system is utilized in the ventilator disclosed in U.S. Pat. No. 4,527,557 with feedback to the controller regarding the flow rate being generated by a flow transducer which is positioned downstream of the flow control valve. The actual flow rate as measured by the flow transducer is compared by the controller on a real-time basis with the preselected flow rate. If a discrepancy exists between these two values, the valve is opened or closed accordingly. During each breath, the microcomputer controller calculates the total volume of gas delivered by incrementally summing the product of the measured flow rate and the length of the incremental time interval. When the volume calculated by the controller equals the preselected tidal volume, the flow control valve is closed.
This system has several drawbacks, most notably being the unreliability of the flow transducer. Currently available flow transducers are expensive, delicate instruments which can provide accurate flow measurements only over a narrow range of flow conditions. Further, a lag time develops between a change in actual flow rate and the system's ability to correct or compensate for the change in flow. As a result, significant differences between the preselected tidal volume and the actual tidal volume delivered can occur.
Another problem with the system's closed loop servo, such as that disclosed in U.S. Pat. No. 4,527,557, is that the control system is complex and difficult to design so that the system is stable. On an unstable system, as the flow control valve is being opened to reach the desired peak flow rate position, flow would be choppy and irregular as the valve overshoots the desired flow rate, and the flow rate oscillates until the valve is able to settle.
Thus, a need exists for an inexpensive ventilator which can accurately and consistently deliver a breath having a preselected tidal volume.